Commonly called the "human plank" trick—yeah, trick—this is a staple of stage hypnotists.
Once an adequate member of the audience has been selected from the pool of volunteers1, three chairs are placed (or are already present) on the stage. The hypnotist does his or her thing, usually in an even, modulated voice and expressive style so that the hypnosis can be heard and seen by the audience, even those in the nose bleed section. Some patter about the power of hypnosis and the human mind and other such business is given. The subject to made to lie across the chairs (often padded for comfort), feet resting on one, head and neck (preferably the top of the shoulders, as well, for safety) on another. The third chair is under the person's "posterior."
Perhaps some more patter about how hypnosis can grant one strength that one cannot normally tap into. The audience and subject are told how the trick about to be performed cannot be done under normal circumstances—circumstances without the benefit of the hypnotist's "art" (at the same time reinforcing in all that is can be done.2
The subject is told that his or her body will become as rigid a plank or a board, allowing the subject to be suspended between only two chairs. Then, either with flourish or deliberate "carefulness" (depending on the choice of showmanship used by the hypnotist), the middle chair is pulled away and to everyone's amazement, the subject is just as promised. All due to the power of the hypnotic suggestion.
Or not. The oft unasked question, is whether it can really be done without benefit of hypnosis. The answer is, of course, yes. It is no serious difficulty for the average (note average, caveats will follow) person to remain in that position for a time, apparently rigid as a plank—hypnotized, fake-hypnotized, or neither.
But that's not all!
To further demonstrate the ability of the power of suggestion, the hypnotist (sometimes an assistant, sometimes another audience volunteer) will stand on the newly created human plank. This truly is amazing—once again, the obvious question being left unasked. Fact of the matter is, a person can briefly stand on the subject (more toward the chest, to better distribute weight. The effect, none the less, is impressive, and most leave with a sense of awe in regard to hypnosis and the power of the human mind.
Caveats. Obviously, this should not go on for more than a few minutes because the subject's neck muscles will tighten and he or she will become uncomfortable (possibly cramp up) from holding the position. Weight restrictions need to be considered, also. It would be a bit more difficult for a very overweight person to hold and maintain the position for any period of time. Also, the standee should not be a very heavy person for obvious reasons. Particularly in relation to the subject (the book cited gives the example of a 300 pound man on a 98 pound woman).
On the other hand, an average man can support up to 300 pounds for a few minutes (always within reason and accounting for the relation of weight between both). Also: take off your shoes before standing on the subject. The book includes a picture of the trick being performed on former Tonight Show host Johnny Carson (appears to be from the 1970s). According to the caption, shortly after it was taken, Bette Midler came over and sat on his stomach.
Another variation is to place a felt pad on the subject's chest and put a block of stone on it. Then the hypnotist breaks the rock with a sledge hammer. The important thing is to use sandstone. The force of the blow is mostly absorbed by the felt and the sandstone (which also breaks apart more easily than, say, granite). This variation is commonly performed without hypnosis or chairs involved. It is essentially identical, the hypnosis version only seems more amazing.
"The Human Plank." Wonderful stagecraft. Entertaining. Hypnosis or the power of the human mind? Nope.
1One should remember that one of the most important things for a hypnotic subject to be is willing to take part in the hypnotist-subject dynamic. In some sense, it is a matter of role-playing as the subject. This, of course, is partly why people do not do things under hypnosis they would not normally do. In the case of a "show" or "performance," the willingness is often increased because of the attention, peer pressure, desire to take part in or help out the show. So, while someone might not normally act odd or silly, in the context of the situation (entertainment and performance; often while out having a good time, perhaps after a few drinks), one's inhibitions for public spectacle are broken down.
That said, this requires no hypnosis to begin with.
2Though no real hypnosis is taking place (or, at least, is necessary), the subject must have confidence in both the hypnotist and himself (or herself). If the subject expects it to fail, that will likely be the result.
Source: Robert A. Baker 1990 They Call it Hypnosis